D. 1815.
At first there were two, but there are now generally three. In this
country and France they are worked by pistons, which, when pressed
down, give access for the air into channels or supplementary tubings
on one side of the main bore, thus lengthening it by a tone for the
first valve, a semitone for the second, and a tone and a semitone for
the third. When released by the finger, the piston returns by the
action of a spring. In large bass and contralto instruments, a fourth
piston is added, which lowers the pitch two tones and a semitone. By
combining the use of three valves, lower notes are obtained--thus, for
a major third, the second is depressed with the third; for a fourth,
the first and third; and for the tritone, the first, second, and
third. But the intonation becomes imperfect when valves are used
together, because the lengths of additional tubing being calculated
for the single depressions, when added to each other, they are too
short for the deeper notes required. By an ingenious invention of
compensating pistons, Mr. Blaikley, of Messrs. Boosey's, has
practically rectified this error without extra moving parts or altered
fingering. In the valve section, each altered note becomes a
fundamental for another harmonic scale.
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